The dependence of the resonance Raman spectra of single wall carbon nanotubes (SWNTs) on an externally applied potential (V-ap) was studied in different media. The spectro-electrochemical data indicate that the reaching of van Hove singularities (vHSs) of SWNTs occurs at a different V-ap for each electrolyte, and that there is a dependence of the charge transfer and intercalation processes on the chemical nature of the media. We present evidence to suggest that the position of the vHSs of SWNTs is perturbed by Vap. Asymmetry between empty and filled vHSs is revealed by the spectroscopic response upon electrochemical p-and n-doping. (C) 2004 Elsevier B.V. All rights reserved.